During the birth process of America, there were two political sides. (By the way, there are always at least two sides, at any time, to any problem.) These two parties did not develop into "political parties" at the end of the eighteenth century, but they were deeply divided on the Conflict with the British Empire, and how to solve it
One group wanted England to treat the citizens of the colonies as British citizens and not as colonialists. The colonialists felt exploited by England. Slowly, a distinct view developed, according to which the English Crown would never treat the colonies with this kind of respect; This group came to the conclusion that the only valid way was to separate from Great Britain and form a new independent country.
The governor of Massachusetts, Thomas Hutchinson, defended the British post. In 1773, he addressed the General Assembly of Massachusetts to argue that the settlements were rightly subject to the authority of Parliament. John Adams refuted his assertions that Parliament did not give the colonists their rights as British subjects. Whatever the case, the question of whether the separation from England was a considerable debate, with numbers on each side, until the first attempt at firing in 1775.
The nation greatly reduced immigration in the years before and after the First World War. In the decades that followed, the percentage of immigrants living in the United States fell, as shown by the red dots on the graph. The blue line shows the degree of polarization in the House, which increased with the increasing percentage of the foreign-born population when the country reopened its borders from the 1960s.
Political parties have always represented classes, regions and industries with divergent interests, who must negotiate to find win-win compromises. But when you look at these trends together, you see that the parties came to represent not only divergent material interests but different types of people with different moral values and ways of life. As these divisions intensified, the Americans came to hate the other party and its members more and more. You can see it in the chart below. Every two years, the US National Electoral Survey asks a representative sample of Americans to say how they feel about many groups and institutions in American life. They give numbers using a scale that goes from zero (very cold, strong aversion) to 50 (neutral) to 100 (very hot, strong taste). The first two bars below show how the Democrats (in blue) feel about the Democratic Party and how the Republicans (in red) feel about the Republican Party. As you can see, these lines show no real trend over time - people generally feel positive about their own party.
But feelings towards the opposite party have tended to decline since 1980, and especially since 2000. Democrats are really not in favor of the GOP and the people who support it. Republicans feel the same about the Democrats. Growing and cross-fertilized hostility involves partisan morality in more and more problems, and it puts pressure on lawmakers not to compromise.
When the Republicans took over the House in 1995, Newt Gingrich made various changes to an institution that Democrats had dominated for 40 years. One of the biggest changes was to encourage new members not to move to Washington, where they were likely to become more moderate in that they (and their families) other side. Gingrich even changed the legislative timetable so that most of the work took place mid-week, allowing members to fly two or three days later.